1. Fix md memory leak:

   mdunlink() and mdclose() (too !!!) now free MdfdVec for relation
   and add it to free list, so it may be re-used for another relation
   later.
2. Fix VFD-manager memory leak (found by Massimo ... and me):
   mdunlink() has to call FileUnlink() to free allocation for fileName
   and add the Vfd slot to the free list.